Field of Invention
An aspect of the present invention relates to an electronic device, and more particularly, to a semiconductor memory device having improved threshold voltage distribution.
Discussion of Related Art
Semiconductor memory devices are generally classified into volatile memory devices and nonvolatile memory devices.
A volatile memory device performs a read/write operation at a high speed, but loses stored data when a power supply is cut off. Alternatively, a non-volatile memory device performs a read/write operation at a relatively low speed, but retains stored data even when a power supply is cut off. Accordingly, the non-volatile memory device is used to store data regardless of interruption of a power supply. Examples of the non-volatile memory device are a read-only memory (ROM), a mask ROM (MROM), a programmable ROM (PROM), an erasable programmable ROM (EPROM), an electrically erasable programmable ROM (EEPROM), a flash memory, a phase-change random access memory (PRAM), a magnetic RAM (MRAM), a resistive RAM (RRAM), a ferroelectric RAM (FRAM), and the like. Flash memories include NOR type flash memories and NAND type flash memories.
The flash memories have an advantage, for example, in a RAM, data is freely programmed and erased. An advantage for a ROM is that stored data is retained even when a power supply is cut off. Therefore, flash memories are widely used as a storage medium for portable electronic devices such as a digital camera, a personal digital assistant (PDA), and an MP3 player.